The present invention concerns methods and devices for heat treating carbon steel wires to obtain a homogeneous austenite structure and, if desired, of subjecting these wires to a subsequent thermal treatment to obtain a fine pearlitic structure.
The known methods of austenitization of travelling steel wires are in particular as follows:
heating by induction, in which the wire is subjected to a magnetic field having a frequency of 5,000 to 200,000 Hz; this method is applied under good conditions only to wires of a diameter larger than 3 mm and at temperatures lower than the Curie point.
heating in a muffle furnace by means of electric resistors; this method avoids the inconveniences of heating by induction, but it requires important heating times on the order of 10 to 15 seconds per millimeter of diameter of the wire to achieve the desired result.
heating in a gas furnace; this method also requires important heating times on the same order as those of the muffle furnace since the temperature of the gases at the outlet of the oven must be low if it is desired to obtain a suitable thermal yield; on the other hand, the thermal conductivity of the combustion gases is not as good as that of the gases which can be used in a muffle furnace (hydrogen, mixture of hydrogen and nitrogen, helium); it is possible in gas furnaces to control the deoxidizing power of the combustion gases, but this requires very careful supervision of the adjustment of the gas burners.